The usual form of a helical resonator filter consists of several helical coils, each wound in the form of a helix, a conductive shell or housing having cavities, each cavity separated by a separating wall from the adjacent cavity and each cavity having a helical coil. The separating wall is apertured to provide an electromagnetic coupling between adjacent helical coils. An inherent characteristic of a conventional helical resonator is that the bandwidth of the filter is determined primarily by the size of the aperture in combination with the input and output coupling to the filter. In other words, the bandwidth that can be provided by the helical resonator filter is set by the geometry of the elements that constitute the resonator.
For tuning the filter, a metal tuning screw is provided into each of the cavities to adjust the capacitive or electrical field of the helical coils. Hence, the tuning of the helical resonator filter is solely a mechanical operation.
While the mechanical tuning operation is acceptable in many applications, it has certain inherent limitations imposed on the filter due to the mechanical nature of the tuning operation. Thus, for example, the tuning bandwidth is essentially defined and limited by physical dimensions of the cavities, helix and size of tuning screw. As a result, the conventional prior art resonator is not capable of shifting the tuned frequencies over a broad spectrum of frequencies.